The invention relates to a High Speed Downlink Packet Access (HSDPA) system and it addresses a method of improving F-DPCH power control performance.
In the Third generation Partnership Project (3GPP) Rel6, a new channel is introduced, F-DPCH (Fractional-Dedicated Physical Channel). The only information sent on the F-DPCH is two transmission power control (TPC) command bits per slot, with spreading factor 256. The downlink power control, for the F-DPCH, can be divided into an inner loop and an outer loop. The outer loop controls the downlink quality in terms of TPC command error rate (CER), by providing signal to interference ratio (SIR) references to the inner loop. The inner loop controls the SIR by sending transmission power control (TPC) commands to the base station. The TPC bits carried on the downlink F-DPCH controls the power transmitted in the uplink and thus the uplink performance. It is therefore essential that enough power is used on the downlink F-DPCH so that the TPC bits are correctly received by a user equipment (UE).
The performance of the SIR estimator and the CER estimator directly impacts the quality of the downlink F-DPCH and therefore indirectly the quality of the uplink channels. Measurement of the CER is not straight forward as there are no traditional pilot bits. This means that there is a need for a method to estimate the CER based on received TPC symbols.
There are a number of known methods for estimating the CER:
1) Based on predefined TPC bits, which simplifies CER estimation considerably, but requires a standard change;
2) Based on SIR estimate, where the mapping between the estimated SIR and the CER is relatively channel independent and which require SIR estimates with high accuracy;
3) By comparing TPC bits, where the mapping between the probability of unequal bits to CER is relatively channel independent; which requires estimating the probability of unequal bits with high accuracy;
4) By considering the ratio of statistical moments of received TPC amplitudes distribution, where the mapping between the statistical moments to CER is relatively channel independent; which is very much dependent on the accuracy of statistical moments.
All the above methods only focus on the UE side to do the F-DPCH power control and to improve the CER estimation accuracy. However, when the UE side controls the downlink power control, there might be an impact on the downlink channel of a bad UE, e.g. some bad user equipment could send “UP” commands with high probability and thus wasting valuable radio resources.